Make tests not create std{out,err}_logfile in cwd

[msabramo]

by modifying 3 tests to use instance.std{out,err}_logfile which are temp files.

This is an alternative solution to the issue mentioned in #382, #377, and #417.

Before:

$ /bin/rm std*_logfile
$ ls -l std*_logfile
zsh: no matches found: std*_logfile
$ tox -e py26
...
$ ls -l std*_logfile
-rw-r--r--  1 marca  staff     0B Apr 14 08:29 stderr_logfile
-rw-r--r--  1 marca  staff     0B Apr 14 08:29 stdout_logfile

After:

$ /bin/rm std*_logfile
$ ls -l std*_logfile
zsh: no matches found: std*_logfile
$ tox -e py26
...
$ ls -l std*_logfile
zsh: no matches found: std*_logfile

Cc: @mnaberez, @mcdonc, @gcarothers, @graffic

[msabramo]

Note that I targeted this PR at master, since it's not Python 3 specific.

[mnaberez]

I haven't looked into this much but it seems like those files started appearing after 2ab5364, but they are created by tests that existed before that change. I'm wondering if this is a sign that something needs to be fixed in that change.

[mnaberez]

Committed in 1f51510.